Again, you're assuming that there only a few entirely orthogonal variables. If A and B are both located in a desirable location, the cost of moving to A or the cost of moving to B might be high enough, such that it's only justified if there is a large increase in salary. So this factor might well apply more to moving from C->A than it would A->B or B->A. The same housing expenses might also motivate other people to move from A->C. In this case, you might well see more A->C->B jumps than A->B.

However, none of these are cleanly applicable to real people in the real world. We're not dealing with particles or spherical cows here.